BOSTON -- A large measles outbreak in the Canadian province of Quebec is raising questions about vaccine effectiveness, researchers said here.

Action Points

Note that these studies were published as abstracts and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

Explain that although the vast majority of cases in the 2011 Quebec province measles outbreak occurred in unvaccinated children, 14% arose in fully vaccinated individuals.

Note that the two-dose immunization schedule in Canada is at 12 and 18 months, while in the U.S. the first dose is administered at 12 or 15 months and the second dose at school entry between ages 4 and 6 years.

BOSTON -- A large measles outbreak in the Canadian province of Quebec is raising questions about vaccine effectiveness, researchers said here.

For the year 2011 up to Oct. 5, the province had recorded 757 confirmed cases, 69% of them in a largely rural region between Montreal and Quebec city, according to Philippe Belanger, MSc, of the Quebec health ministry.

But all regions of the province have been affected, with an overall incidence rate of 9.5 cases per 100,000 people, Belanger said during a late-breaker session at the annual meeting of the Infectious Diseases Society of America.

In the hardest-hit region, however, the attack rate was 105.6 per 100,000 people, Belanger said.

What is striking about the outbreak is that – while it is mainly affecting unvaccinated people – 14.1% of the cases involved people with documented two-dose vaccination, Belanger said. Another 9.8% of the cases involved people who could show they had one dose of the vaccine, with another 9.4% among those who said they had been vaccinated but could not prove it.

"It is not clear that this is only a failure to vaccinate," he said.

Interestingly, of the 101 cases among people with documented complete vaccination, 80 -- or 79% -- were ages 10 to 19, Belanger reported. Among the 468 patients known not to be vaccinated, the clustering was smaller, with 65% of cases among those ages 10 to 19.

One implication of that may be that the vaccine is less effective than had been thought, especially in those age groups, according to Gaston De Serres, MD, PhD, of the Quebec national institute for public health.

De Serres presented a second paper during the session, analyzing the outbreak in the hardest-hit region and in particular looking at a single high school that accounted for 15% of the cases in the region.

That outbreak was apparently sparked by a single teacher who brought the virus home from a Caribbean vacation, De Serres said. All told, the school recorded 98 classical cases and another 12 cases that met some but not all of the criteria for a measles case.

Only 4.7% of the students in the school were unvaccinated, he noted, and they bore the brunt of the outbreak, with half of the 98 cases and an attack rate of 82%. On the other hand, there were 41 cases among the 1,111 students with complete vaccination, for a much lower attack rate of 3.7%.

But that is still higher than would be expected, De Serres said, if the two doses of vaccine – delivered at 12 and 18 months – are as effective as has been thought.

Analysis showed that the attack rate was 5.9% if students got their first shot at 12 months, with a vaccine efficacy of 93%. But among the minority whose first shots were delayed to 15 months, the attack rate was 2.1% with a vaccine efficacy of 97.5%, suggesting that the later vaccine schedule prolonged immunity.

The implication, De Serres said, may be that the immunization schedule used in Quebec and most other Canadian provinces – expected to provide greater than 99% protection – is inadequate. He said the analysis needs to be replicated in jurisdictions – such as the U.S. -- where the second dose is given later.

It's difficult to pinpoint why the Quebec outbreak is so large, especially when – even in a very high season – U.S. outbreaks have had a median size of about five cases, according to Andrew Pavia, MD, of the University of Utah, who was not part of the research.

Part of it, he told MedPage Today, might be that the U.S. has a higher rate of people with two doses or more – greater than 95%, compared with about 89% among Quebec two-year-olds and 92% among those 14 and 15 years old.

Another possible factor, he said, may be a difference in immunization schedules, as De Serres suggested. In Quebec, babies usually get their first dose at a year and the second dose six months later. In contrast, most U.S. children get the second dose at school entry, which might give them a better immunity in their high school years.

Many adults in the U.S. also get a third dose when they enter college, he noted.

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